Anisotropy of the Taylor scale and the correlation scale in plasma sheet magnetic field fluctuations as a function of auroral electrojet activity

WEYGAND, J. M.; MATTHAEUS, W.H.; EL-ALAOUI, M.; DASSO, S.; KIVELSON M.G.

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2010 vol. 115 p. 15449 - 15459

0148-0227

Magnetic field data from the Cluster spacecraft in the magnetospheric plasma sheet areemployed to determine the correlation scale and the magnetic Taylor microscale fromsimultaneous multiple‐point measurements for multiple intervals over a range of meanmagnetic field directions for three different levels of geomagnetic activity. We havedetermined that in the plasma sheet the correlation scale along the mean magnetic fielddirection decreases from 19,500 ± 2200 to 13,100 ± 700 km as the auroral electrojetactivity increases from quiet (<80 nT) to active conditions (>200 nT). The reverseoccurs for the correlation scale perpendicular to the magnetic field, which increases from8200 ± 600 km to 13,000 ± 2100 km as the auroral electrojet activity increases fromquiet to active conditions. This variation of the correlation scale with geomagnetic activitymay mean either a change in the scale size of the turbulence driver or may mean achange in the predominance of one over another type of turbulence driving mechanism.Unlike the correlation scale, the Taylor scale does not show any clear variation withgeomagnetic activity. We find that the Taylor scale is longer parallel to the magneticfield than perpendicular to it for all levels of geomagnetic activity. The correlation andTaylor scales may be used to estimate the effective magnetic Reynolds numbers separatelyfor each angular channel. Reynolds numbers were found to be approximately independentof the angle relative to the mean magnetic field. These results may be useful inmagnetohydrodynamic modeling of the magnetosphere and can contribute to ourunderstanding of energetic particle diffusion in the magnetosphere.